If you go to the mail queue page, it says this:

Mail Queue
This page shows you the current JIRA internal event queue, whose events may trigger notification emails. Note: it is only possible to delete a screen if it is not part of a Screen Scheme and is not used in any workflows.

OK. But what does deleting a screen have to do with a mail queue??? Maybe it was to have been added for another screen.

On 3.6.5 that note isn't there